'Cracking Shot' I hear you say.

The speed a crack propagates is related to the diameter at the very point of propagation and the brittleness of the material. The reason that breaking glass has such a distinctive loud noise when it breaks is that it is very brittle and diameter of propagation is microscopic - the crack propagates at a speed that exceeds the speed of sound. It is also the reason broken glass is incredibly sharp.

On that premise - if you have a crack meandering across a ceiling don't just polyfiller it. Find the very point of the crack and drill a 1/4" hole thus increasing it's diameter greatly.then polyfiller it. Crack stopped.

Just felt the need to share that with you, back to the usual pish tomorrow.